1. Field of the Invention
The present invention is related to a self-loading type of controlled deflection roll, as is used in the papermaking industry, which forms one roll in a pair of nipped rolls for processing a traveling web of paper.
2. Description of the Prior Art
Pairs of rolls forming a nip through which a traveling web passes are used at many locations in a papermaking machine, particularly in the press section to mechanically remove water from the web. In such nips, one or both rolls are loaded, i.e., the roll is mechanically forced toward the nip in order to exert a desired amount of pressure on the web as it travels through the nip. It is also necessary to be able to mechanically retract the rolls of a nip away from each other, so as to open the nip. Such retraction is necessary not only to be able to control the nip pressure, but also as part of the start-up procedure for the papermaking machine either at the beginning of a new production run, or after a sheet break. The start-up procedure involves the cutting and threading of a "tail" through the machine. The papermachine is usually, but not always, threaded at or near operating speed. The speed may be increased or decreased during operation, after threading. During this start-up procedure, a nip will not be loaded at its normal operating pressure. For many years in the papermaking industry, loading of rolls was accomplished by suitable mechanisms disposed at one or both ends of the roll shaft about which the roll rotates. Such mechanisms moved the entire roll on its shaft toward and away from the mating roll in the nip.
In order to provide uniform processing of the entire width of the web in the cross-machine direction as it travels through a nip, it is desirable to have the line of contact between the two rolls forming the nip be as straight as possible or, if one of the rolls has a contour which is not a straight line, to have the other roll follow that contour as closely as possible. As improving technology in the papermaking industry permitted papermaking machines to be made increasingly wider in the cross-machine direction, as well as to operate at increasingly faster speeds, the sheer weight of the roll or the roll shell, supported only at its opposite ends, resulted in a slight "sag" of the roll in a central region of the nip, thereby causing the line of contact between the two rolls in a nip to exhibit a non-uniform distance between the rolls along the cross-machine direction.
Controlled deflection rolls were developed in response to this problem. The first generation of such controlled deflection rolls were intentionally loaded at their opposite ends so as to cause the roll shell to exhibit a slight outward bow in opposition to the aforementioned sag, so that the distance between the two rolls in the nip would be uniform along the entire cross-machine width of the nip.
More recently, so-called self-loading controlled deflection rolls have been developed, wherein a number of hydraulically operated shoes are carried on a center shaft disposed inside the roll shell, the shoes being actuatable to move toward and away from the axis of rotation of the roll, so as to push against the inner surface of the roll shell, thereby achieving the desired deflection of the outer surface of the roll shell. The need to provide complicated mechanisms at the opposite ends of the roll to move the roll toward and away from the nip is thereby avoided, and only mechanisms for rotating the roll need to be provided at one or both ends, typically only at one end. Examples of such self-loading controlled deflection rolls are disclosed in U.S. Pat. Nos. 5,193,258, 5,127,141 and 5,111,563.
Such self-loading controlled deflection rolls typically have a hydraulically operated center shoe disposed at a central region of a support shaft extending through in the interior of the roll shell, as well as front and back shoes respectively disposed at the front end and the back end of the shaft inside of the shell. The shoes are moved radially outwardly and inwardly (relative to the rotational axis of the roll) by means of hydraulic fluid, such as oil, which is delivered through the central shaft to the shoes. Typically, delivery of the hydraulic fluid to the shoes at the opposite ends of the roll takes place by means of a conduit system supplied from a source of hydraulic fluid at one end of the roll. This means that one shoe will be closer to the source of hydraulic fluid than the other shoe, with a hydraulic fluid line running between the shoes. As a result of the unavoidable pressure differential which is present in the hydraulic line running between the shoes, as well as due to other factors, the pressure exerted by the hydraulic fluid on one of the front or back shoes will be slightly different from the pressure exerted by the hydraulic fluid at the other shoe at the opposite end of the roll, thereby causing the shoes to be displaced by respectively different amounts, and thereby causing uneven radial movement of the roll shell. Such uneven radial movement is undesirable because nip engagement is then non-uniform, which can lead to threading problems and sheet breaks. Moreover, if one end of the shell is radially displaced by a different amount from the other end, this results in the shell being out of parallel with the center shaft axis. This causes the internal portions of the shoes to become misaligned within the center shaft piston groove in which they move. For the purpose of accommodating such misalignment, conventional self-loading controlled deflection rolls employ end dam seals. Such misalignment also results in the rotary oil seals and other internal parts being subjected to misalignment as well, which increases the wear to which all of these parts are subjected, and thus decreases the useful life of those parts. Papermaking machines are such a large investment that, ideally, a mill operator would like to operate the machines in a manner as closely approaching continuous operation as is possible, with down time being maintained to a minimum. To the extent that the above-mentioned misalignment problems can be avoided, or minimized, and thus reduce the wear on the parts, and if such misalignment can be minimized or avoided without significantly adding to the complexity of the roll structure, down time will be similarly reduced, thereby increasing the profits of the mill operator.
Position sensors can be placed at suitable locations within the roll so as to detect misalignment when it occurs, however, such position sensors, and the associated control circuitry, are costly, and require frequent maintenance and adjustment, and therefore even though they may reduce the wear on the parts and thus reduce the down time associated with replacement of those parts, the position sensors and control circuitry themselves have a certain amount of down time associated therewith and are thus not an optimum solution. Alternatively, separate hydraulic delivery systems could be respectively provided for each of the end shoes, however, this would involve a duplication of equipment, and would also require means for ensuring that the flow in each delivery system was identical.